This invention relates to a novel catalyst that is suitable for use in the homopolymerization of norbornenes, in the homopolymerization of acrylates, and in the copolymerization of norbornenes with acrylates. The invention relates, further, to a process for homopolymerizing norbornenes and acrylates using the present catalyst, to a process for copolymerizing norbornenes with acrylates using the present catalyst, and to novel copolymers of norbornenes with acrylates.
Considerable interest has existed in the copolymerization of acrylates with norbornenes because of the potential benefits of combining the useful properties of the homopolymers of the two monomers. For example, polyacrylates are valued for their extreme hardness and adhesive properties, and are used to form clear, glass-like materials such as Lucite(trademark) and Plexiglas(trademark). Polynorbornenes, on the other hand, are capable of resisting high temperatures and, thus, typically are employed in applications that necessitate high-temperature stability.
Polymerization of acrylic derivatives is disclosed, for example, in U.S. Pat. No. 4,849,488. In that patent, certain phosphorus compounds are used as polymerization catalysts. Preferred acrylic derivatives that are polymerized in accordance with that patent are acrylates and methacrylates of monovalent and polyvalent alcohols. Copolymers of at least two different acrylic derivatives are also disclosed. There is no disclosure in that patent relevant to the possibility of copolymerizing an acrylic derivative with norbornene or a norbornene derivative.
Until the present invention, attempts to copolymerize acrylates with norbornenes have met with modest success because of a disparity in the mechanisms by which the respective monomers polymerize. U.S. Pat. No. 3,697,490, for example, discloses copolymers of alkoxy alkyl acrylates, alkyl acrylates and substituted norbornene, wherein the copolymers contain only about 0.5 to about 5.5 percent by weight substituted norbornene compounds. The copolymers described in that patent typically are prepared in batch reactions, or the monomers may be proportioned to a reactor containing water and other desired polymerization additives. The patent indicates that best results are generally obtained at polymerization temperatures in the range of 5xc2x0 C. to 50xc2x0 C. in the presence of water containing a free radical generating catalyst and surface active agents.
U.S. Pat. No. 6,034,259 broadly discloses a process for polymerizing ethylene, acyclic olefins, and/or selected cyclic olefins, and optionally selected olefinic esters or carboxylic acids. The polymerization reactions are catalyzed by selected transition metal compounds, and sometimes other co-catalysts. Also described in that patent is the synthesis of linear alpha-olefins by the oligomerization of ethylene using as a catalyst system a combination of a nickel compound having a selected alpha-diimine ligand and a selected Lewis or Bronsted acid, or by contacting selected alpha-diimine complexes with ethylene. That patent also discloses polymerization of substituted norbornenes using the disclosed alpha-diimine-containing catalysts systems.
Japanese publication JP040063810 relates to copolymers that contain units derived from three essential monomers, namely: (i) an acrylic ester and/or methacrylic ester monomer, (2) a monomer that possesses a norbornene skeleton, and (iii) a monomer that can be radical-polymerized with the monomer that contains the norbornene skeleton. The copolymers must contain from about 0.5 to about 35 mole % of units derived from radical-polymerizable monomer (iii). The compounds that are disclosed as being suitable for use as the monomer (iii) include esters of maleic acid and an aliphatic alcohol which contains from 1 to 12 carbon atoms, maleic anhydride, N-substituted maleimides, xcex1-cyanocinnamic acid, esters of xcex1-cyanocinnamic acid and an aliphatic alcohol which contains from 1 to 12 carbon atoms, and esters of fumaric acid and an aliphatic alcohol which contains from 1 to 12 carbon atoms. The copolymerization reaction that is described in the Japanese publication typically is conducted in the presence of a free radical polymerization initiator, such as acetyl peroxide, benzoyl peroxide, 2,2xe2x80x2-azobiscyclopropylpropionitrile, or the like.
Typically, acrylates polymerize in the presence of radical or anionic initiators, whereas norbornenes do not follow radical pathways and normally are polymerized by cationic or insertion mechanisms. Therefore, in order to affect the copolymerization of acrylates with norbornenes, it was necessary to develop a catalyst system that would be effective for polymerizing both types of monomers.
It is an object of the present invention to provide a catalyst system that is capable of copolymerizing acrylates with norbornenes.
It is another object to provide a catalyst system that is useful for the homopolymerization of both acrylate monomers and norbornenes.
It is yet another object to provide a novel catalyst system that is useful both for the homopolymerizing of acrylate monomers and norbornene monomers, and for the copolymerization of acrylates with norbornenes.
Still another object of the invention is to provide novel copolymers of acrylates with norbornenes.
The above and other objects and advantages of the invention are accomplished in one embodiment by providing a Pd(II)-based catalyst system which homopolymerizes acrylates to high molecular weight polymers, which homopolymerizes norbornenes to polymers, and which copolymerizes acrylates with norbornenes to high molecular weight polymers. The copolymers prepared in accordance with this invention are characterized by advantages and properties attributable to the respective acrylate and norbornene monomers from which they are derived. The copolymers can be tailored in norbornene to acrylate ratio by varying the ratio of the respective monomers in the reaction mixture and by varying the ligands utilized in the catalyst system.
The copolymers can be prepared simply by reacting one or more acrylate monomers with one or more norbornene monomers in the presence of the present Pd(II)-based catalyst system. The acrylate and norbornene monomers are the only essential monomers, although additional monomers may be added to the polymerization mixture, provided that they do not interfere with the desired polymerization. Thus, while there may be some instances where it would be desirable to add monomers that can be radical-polymerized with the norbornene monomer, such as the monomers described above in connection Japanese publication JP040063810 as radical-polymerizable monomer (iii), it is preferred to prepare the copolymers of the present invention in the absence of such added monomers.
As used in this specification and claims, the terms xe2x80x9cacrylatesxe2x80x9d is meant to include compounds of the general formula H2Cxe2x95x90CHCOOR, where R is an alkyl group, such as methyl (CH3), ethyl (CH2CH3), propyl (CH2CH2CH3), n-butyl (CH2CH2CH2CH3) or t-butyl (Cxe2x80x94(CH3)3), or an aryl group, such as phenyl (C6H6) or p-tolyl (C7H8). Other acrylates which do not conform to the above formula, but which are nonetheless suitable for use in the present invention and are intended to be included within the scope of the term xe2x80x9cacrylatesxe2x80x9d, include such acrylates as 2-hydroxy ethyl methacrylate and methyl methacrylate. Specific, non-limiting examples of acrylates contemplated for use in the present invention include methyl acrylate, ethyl acrylate, n-propyl acrylate, n-butyl acrylate, t-butyl acrylate, acrylamide, 2-hydroxy ethyl methacrylate, methyl methacrylate, acrolein, ethyl methacrylate, acryloyl chloride and phenyl acrylate.
The term xe2x80x9cnorbornenesxe2x80x9d is meant to include norbornene, as well as norbornene derivatives, such as norbomadiene and compounds conforming to the general formulas 5-norbornene-2-R1a, 5-norbornene-2,2-R1a, 5-norbornene-3-R1a, 5-norbornene-3,3-R1a, 5-norbornene-2,3-R1a, or 5-norbornene-2-R1a-3-R1b, where R1a and R1b, independently, represent an alkyl or aryl group. Specific, non-limiting examples of norbornenes contemplated for use in the present invention include norbornene, 5-norbornene-2-methanol, cis-5-norbornene-endo-2,3-dicarboxylic anhydride, 5-norbornene-2,2-dimethanol, 5-norbornen-2-ol, norbornadiene, 5-norbornene-2,3-diphenyl, cis-5-norbornene-endo-2,3-dicarboxylic acid dimethyl ester, 5-norbornen-2-yl-acetate and 5-norbornene-2-carboxyaldehyde.
The Pd(II)-based catalyst system of the present invention is a dimer catalyst that may be characterized by the formula [(L)Pd(R)(X)]2. In this dimer catalyst, L is a monodentate phosphorus or nitrogen ligand. In general, suitable monodentate phosphorus ligands are those that conform to the formula R23P, where R2 is alkyl and/or aryl group. Non-limiting examples of such monodentate phosphorus ligands include triphenyl phosphine (PPh3), tricyclohexyl phosphine (PCy3), trimethyl phosphine (PMe3), triethyl phosphine (PEt3), tri-n-propyl phosphine (P(n-Pr)3), tri-n-butyl phosphine (P(n-Bu)3), tri-t-butyl phosphine (P(t-Bu)3) and tri-p-tolyl phosphine (P(p-Tol)3). Specific, non-limiting examples of mixed alkyl and aryl phosphines that are suitable for use in the invention include methyldiphenyl phosphine (P(Me)(Ph)2), phenyldimethyl phosphine (PPh(Me)2), ethyldiphenyl phosphine (P(Et)(Ph)2), and phenyldiethyl phosphine (PPh(Et)2).
Generally, monodentate nitrogen ligands which may be used in the present invention are aromatic or heterocyclic amines, such as pyridine, t-butyl pyridine, aniline, trimethyl aniline and imidazole, or compounds that conform to the formula R3N, where R3 is an alkyl group, such as methyl (CH3), ethyl (CH2CH3), propyl (CH2CH2CH3), n-butyl (CH2CH2CH2CH3), or t-butyl (Cxe2x80x94(CH3)3), or an aryl group, such as phenyl (C6H6) or p-tolyl (C7H8).
X is an anionic group, such as chlorine or bromine, or a carboxylate, such as acetate, propionate, trifluoroacetate, and benzoate; and R is an alkyl group, such as methyl (CH3), ethyl (CH2CH3), propyl (CH2CH2CH3), n-butyl (CH2CH2CH2CH3), or t-butyl (Cxe2x80x94(CH3)3), or an aryl group, such as phenyl (C6H6) or p-tolyl (C7H8).
Typically, the catalyst is formed by reaction of 1 equivalent of the monodentate ligand with [(1,5-cyclooctadiene)Pd(Me)(X)] in situ, as illustrated by the equation (using X=chlorine):
Catalyst formation may be monitored by 1H and 31P{1H} NMR. For the PCy3 analog, the movement of the methyl signal in the proton spectrum from a singlet at 1.12 ppm in the starting material to a triplet at 0.11 ppm in the dimer upon addition of 1 equivalent PCy3 indicates the formation of the desired product. Additionally, the appearance of uncoordinated 1,5-cyclooctadiene signals at 5.55 and 2.34 ppm in the proton NMR and a 25.6 ppm 31P{1H} NMR signal for [(PCy3)Pd(Me)(Cl)]2 are observed.
The relevance of the alkyl (or aryl) group in the present dimer catalyst system was explored by attempting to homopolymerize methyl acrylate using a dimer catalyst that was devoid of alkyl (or aryl) groups. It was found that methyl acrylate would not polymerize when reacted in the presence of a Pd(II) catalyst that was prepared by reacting [(1,5-cyclooctadiene)Pd(Cl)2] with 1 equivalent of tricyclohexyl phosphine (PCy3). This indicated that the alkyl (or aryl) group is essential to the polymerization mechanism.
The copolymers of the present invention may be tailored in norbornene to acrylate ratio by varying the ratio of the respective monomers in the reaction mixture and by varying the ligands utilized in the catalyst system. Typically, the ratio of norbornene monomer to acrylate monomer in the starting mixture is from about 100:1 to about 1:100. In preferred aspects of the invention, the ratio of norbornene monomer to acrylate monomer that is added to the reactor is from about 10:1 to about 1:10. By varying the ratio of the norbornene monomer to acrylate monomer in the starting mixture copolymers having a very wide range of acrylate-derived units and norbornene-derived units can be prepared. For example, copolymers containing from about 5 to about 95 mole % acrylate-derived units and from about 95 to about 5 mole % norbornene-derived units may be prepared in accordance with this invention. In certain preferred aspects, copolymers containing from about 15 to about 90 mole % acrylate-derived units and from about 85 to about 10 mole % norbornene-derived units are prepared; and in other preferred aspects, copolymers containing from about 25 to about 80 mole % acrylate-derived units and from about 75 to about 20 mole % norbornene-derived units are prepared. In still other preferred aspects, copolymers containing from about 35 to about 55 mole % acrylate-derived units and from about 45 to about 65 mole % norbornene-derived units are prepared.
The polymerization preferably is carried out in the liquid phase using a solvent, such as dichloromethane (CH2Cl2), benzene (C6H6), chlorobenzene (C6H5Cl) or hexane (C6H14). Other solvents that may be used as the polymerization medium include, for example, pentane (C5H12), toluene (C7H8), and chloroform (CHCl3).
The polymerization in accordance with this invention may be carried out at temperatures ranging from about 0 to about 200xc2x0 C. Typically, however, the polymerization will be carried out at a temperature of from about 30 to about 80xc2x0 C., e.g., about 50xc2x0 C. The pressure at which the polymerization is carried out is not critical.
The various aspects of the invention will be appreciated more fully in light of the following illustrative examples: